Wireless data services are used for remote access of e-mail, mobile Internet browsing, company information and others. To attract new and existing subscribers to data services, the operator need to support perceived data rates of 64 kbps or greater. Within GSM networks, two such services are General Packet Radio Service (GPRS) and Enhanced Data for GSM Evolution (EDGE). Both bring packet data connectivity to the GSM market at a variety of data rates. To support the high data rates in either service, however, requires increasing the signal-to-noise ratio (SNR) at the receiving handset. As a handset user travels further from a transmitting base station, the SNR must inevitably drop. For both GPRS and EDGE receivers, as the SNR drops, the bit error rate at the receiver will increase. In response, both services will shift the data transmission to a lower data rate, using more redundancy (channel coding) in the transmission to offset the lowered SNR. Thus, users will find that their realized data rate will decrease by a significant percentage when moved from locations near a cell base station toward the cell edges due to increased channel interference (C/I) and reduced signal-to-noise ratios (SNR). Typically, less than 20% of a cell area is estimated to be capable of the peak GPRS data rate in a coverage limited deployment.
One way to increase the data rates for these services would be to locate RF repeaters within a cell that retransmit the base station signals to handsets remote from the cell's base station. Similarly, the RF repeater could retransmit handset signals to the cell's base station. An inherent problem of repeating a signal transmission with little or no frequency translation is that the retransmitted signal may feedback into an RF repeater's receiving antenna. This unstable feedback problem is analogous to the familiar “howl” heard at public events when a speaker's microphone is too close to the loudspeakers. One way to combat this feedback problem would be the use of directional antennas in the RF repeater. For example, one directional antenna would be directed towards the base station and another directional antenna would be directed towards the mobile unit. Ordinarily, however, the RF repeater would not know the position of the mobile unit and would thus use an omni-directional antenna to communicate with the mobile unit. Moreover, even with the use of directional antennas, near field effects are very unpredictable and could cause substantial feedback. In contrast, frequency translation provides better isolation—the RF repeater would transmit and receive signals with the mobile unit on a different frequency band than that used to transmit and receive signals with the base station.
U.S. Pat. No. 5,970,410 discloses a wireless system architecture employing RF signal repeaters using frequency translation to permit a single home base station to communicate with mobile stations in a plurality of cells. Although this reference discloses the use of frequency translated repeaters [they are referred to as “translator base stations” but do not perform the conventional demodulation and modulation functions of a base station], the repeaters are used to extend the coverage of a home base station to additional cells. No teaching is made to use translators to increase the throughput of wireless communications within a given cell. U.S. Pat. No. 5,787,344 discloses an array of repeaters arranged in respective cell areas about a base station within a given cell.
The repeaters in the array may each be allocated different transmission frequencies. Such an arrangement of frequency translating repeaters is complicated if a transmission frequency of the base station is frequency hopping over time. If the signal to be repeated is not frequency hopped, the repeaters could be identified by their particular transmission frequency. If however, the repeaters had to repeat a frequency hopped signal, a given mobile unit would have to know what frequency hopping pattern would be followed by the repeater it is currently communicating with. This, in turn, would require a means for the mobile unit to identify a particular repeater within the array of repeaters. However, no teachings were made in U.S. Pat. No. 5,787,344 for enabling such an identification.
If RF signal repeaters are used to increase the throughput of wireless communication in a given cell, care must be taken not to interfere with the conventional cellular telephony functions. In particular, use of an RF signal repeater will interfere with estimating the location of a mobile unit—wireless networks are currently being enhanced to provide position measurement capability. These measurement methods provide the network with an estimate of the location of a mobile unit, such as for emergency 911 (E911) mobile phone service. Location Measurement Units (LMU) may be inserted either at the base station or at the mobile unit with time of arrival (TOA) signal processing means to estimate the distance from the mobile unit to the base station, assuming straight-line radio propagation.
Such straight-line radio propagation would not be present if RF repeaters are used to boost the mobile transmitted signals at the periphery of the coverage area. For example, a handset may be 2 miles due north of an RF repeater that in turn is 2 miles due east of a base station. The straight line distance between the handset and the base station is the hypotenuse (2.84 miles) of the right triangle formed by the handset, RF repeater, and base station. However, the actual path taken between the handset and the base station would be 4 miles: the 2 miles between the handset and the RF repeater and the 2 miles between the RF repeater and the base station. The use of an RF repeater thus results in significant location measurement errors for the repeated mobile signals.
One method for solving the erroneous E911 LMU distance calculation problem would be to co-locate an LMU next to each RF repeater along with a means for communicating the distance measurement information back to a network-based mobile location center. This method is inefficient in at least two ways: first, it requires an LMU at every repeater site which adds to costs and makes each repeater site bulkier; and second, a separate data link or additional data bits are needed to transmit the distance information to the base station.
Accordingly, there is a need for RF repeaters with the ability to enhance signal to noise and interference ratios over selected areas within a cell site, in particular to enable high data rate transmissions required by a wireless data system without interfering with an efficient location estimation of a mobile unit.